Based on increasing requirements of users on bandwidth, a PON (Passive Optical Network) system, which is extremely competitive, of an optical fiber access network that is used as a new generation broadband access network becomes an object mainly used by the users. Generally, the PON system includes one OLT (Optical Line Terminal, optical line terminal) located in a central office, one ODN (Optical Distribution Network, optical distribution network) used for bifurcation/coupling or multiplexing/demultiplexing, and a plurality of ONUs (Optical Network Unit, optical network unit).
However, with increasingly faster development of various broadband services, such as a video conference, a 3D television, mobile backhaul, an interactive game, and a cloud service, a line rate in the PON system also needs to be improved constantly. In particular, the existing EPON system and GPON system need to be gradually upgraded to high-speed (a speed of 10 G or above) PON systems. However, for a PON system of a high modulation rate of 10 G or above, an obvious dispersion penalty is caused even within a transmission distance of 20 km, causing that quality of a transmit signal is seriously degraded and system receiving sensitivity is seriously reduced. A dispersion penalty of transmission is closely related to a transmission and modulation manner of the PON system, different modulation schemes cause different frequency chirps of an optical signal, and this directly causes different introduced dispersion penalties. Currently, an OLT-side optical module of a PON system of 10 G high-rate modulation mainly uses: a transmission and external modulation manner of an EML (Electro-absorption Modulated Laser) and a transmission and direct modulation manner of a DML (Directly Modulated Laser, directly modulated laser).
The transmission and external modulation manner of the EML refers to that: an output light of a laser is directly injected into an external modulator, a modulation signal controls the external modulator, and an acousto-optic effect and an electro-optic effect are used to enable a parameter, such as intensity of output light of the external modulator, to change with the signal. Because the laser works in a static direct current state, an output signal has a small frequency chirp and high transmission performance. Therefore, at the present stage, 10 G PON OLT-side optical modules all use a transmission manner of an EML (Electro-absorption Modulated Laser, electro-absorption modulator). However, a cost of the EML is high, and an insertion loss is greater (6 dB to 8 dB), causing that transmission power consumption remains at a high level.
The transmission and direct modulation manner of the DML refers to modulating output of a semiconductor laser by changing an injected current, a structure of the DML is simple and is easy to implement, and a cost is low. However, a modulation current causes a change in an active layer refractive index, causing that a phase of a light is modulated, and therefore, an operating frequency is broadened, that is, a relatively great frequency chirp exists, and with increase of a modulation rate, a chirp phenomenon becomes more serious. Therefore, it is difficult to directly use the traditional DML as a transmitter of a high-speed OLT optical module of 10 G or above.
Therefore, an optical transmitter with a high modulation rate that is based on a DML technology, has a low cost, consumes less power, and can reduce the chirp phenomenon is urgently needed, so as to resolve engineering problems currently facing the 10 G-PON and facing a higher-rate PON system in the future.